Nickel-chromium-molybdenum alloy



U t St 1 SPatent 2,840,469 NICKEL-CHROMIYUM-MOLYBDENUM ALLOY Harold E. Gresham, Little Eaton, Marcus Alan Wheeler,

. Darley Abbey, and Jack Raymond Bird, Allenton, England, assiguors to Rolls-Royce Limited, Derby, England, a corporation of Great Britain g No Drawing. Application June 2, 1955 Serial No. 512,876

Claims priority, application Great Britain June 18, 1954 8 Claims. 01. 1s-171 This invention relates to a nickel-chromium-molybdenum base alloy. I

Claim is made for the benefit of the filingdate of patent application No. 18040/54 filed in Great Britain the 18th day of June 1954. p

As the power of the gas-turbine engine is raised by using higher gas temperatures, existing alloys for both the rotating turbine blades and the nozzle guide vanes or static blades not only have a shorter creep life but, owing to the steeper time-temperature rise, are liable to fail by cracking after a limited number of thermal cycles before their staticcreep life is expended at the higher temperature. The latter failure is because the temperature gradients on both bladesand ,vanes on heating and cooling cause very severe stress conditions in these high powered engines.

The latter form of stressing, more commonly referred to as Thermal Shock, has become a very serious prob lem. The edges of the blade or vane in a modern gasturbine engine in starting up can .be raised in a matter of seconds from cold to temperatures in the order of 900 C. on the blade and 1200 C. on the vane while the central portion remains relatively cool and, on shut down of the engine, the edges are cooled again almost as quickly while the central portion remains hot.

An alloy which has very good creep strength usually has poor thermal shock resistance and one which has satisfactory thermal shock resistance usually has poor creep strength. No alloy is known to us which is easy to such index of about '70, and an'hours to fracture under the stated conditions of about 350,-each substantially higher than the corresponding property of any known alloy. Consequently, we believe we have discoveredan alloy which can be used equally well in either blades or vanes, and which is better in either service than the alloyspreviously used for thatpurpose. p 7 According to the present invention in its broadest sense, the alloy consists by weight of about 18' to 25% chromium, about 8.5 to" 12% molybdenum,"-ab'out 0.20 to 0.85% carbon, about 5 to 15% cobalt, and the balance essentiallynickel, except for impurities. I

The effect of variations in the amounts of'the 'fore going ingredients, according to our present information,

is as followsz a Chr0mium.-The corrosion and creep resistance of the alloy is best when the chromium is between about 20% and 23%. As the amount of chromium. is reduced below 20%, there is ,a progressive falling off incorrosion and creep resistance. When the amount of chromium I has been reduced to about 18%, the alloy is no longer sufliciently resistant to be acceptable for the purposes mentioned. Increasing the amount of chromium above 23% appears to be without detriment to the qualities of the alloy. Since however chromium appear'sto serve no useful functionin amounts larger than about-23 to.

a 25%, we do not recommend its use in,larger amountsI cast, and in both the as cast and the forged conditions stress at slightly lower temperatures,.the mostimportant 7 property to be emphasized is creep strength; On the other hand, this particular species ofi'alloy cannot'be satisfactorily used in the vanes Where, becauseof the higher temperatures of operation, a different species'of the above alloy has been developed to provide adequate resistance to thermal shock. Thus, two ditferentalloys had heretofore been required for blades and vanes.

In the United States it is common practiceto cast both blades and vanes in cobalt base materials such as those known under the name Stellite, the compositions of which are summarized below. I

An alloy according to the present invention possesses a thermal shock index at 950 C. of at least about and an hours to fracture characteristic (creep strength) under stress of 4480,1bs. per square inch at 1050 C. of

Molybdenum.0ur alloy appears to be at its best when the molybdenum is about 9.5 to',11%.fAs'.th'e amount of molybdenum is reduced below about 9.5%,

g the creep-strength falls off progressive1Y.:- Ifjthe molyb- 1 denum is less than about 8.5%, the alloy isunaCCeptabIe for use in turbine blades because its.creep strength, is less than that of other alloys. We do not find thatany significant advantage (or detriment) isproducedin the properties of our alloy by increasing the molybdenum above 11 to 12%, and therefore donot recommendits use in larger amounts.

v Carbon.The highest creep strength is attained when the carbon is in. the approximate range 0.30 to 0.50%,

combined with-optimum amounts of cobalt as pointed out below. a A reduction of carbon below about-0.30% produces detectable reduction in the thermal shock and creep properties of the alloy, andwhen carbon is below about 0.20% these properties are so far reduced, that the alloy is no longer useful forits intended purpose. Likewise, an increase of carbon from about 0.50 to 0.85%, with the same amount of cobalt, produces a progressive deterioration of the thermal shock andcreep properties, and when the carbon exceeds about 0.85% these properties have fallen ofi so far that the alloy is no longer usefulfor its intended purpose.

Cobal t .-The best results are obtained using cobalt within the range of about 9 to 11%,IWithlcarbon" held constantin the optimum: portion of its range. As the amount of cobalt is progressively reduced from about 9 to 5%, there is a;slightibutaprogressive reduction in the creep strength ofithealloygand'when the amount of co- .balt is reducedbelow about 5% .the creep properties are such that the alloy is no longer useful for its intended purpose. Increase of cobalt from about 711 to 'l5% does not appear to cause any significant improvement (or detriment) in the properties of the alloy. Since cobalt is Patented June 24, 1958 about: 0.3%,: iron should not exceed-,abojut' 0.75%, and.

residual; traces of cleansersand'deoxidisers such as silicon-andrnan'ganese should not" exceed about.1%.. Fur-. thermore, as will. be understood :by; experts" in this art, all oflgthe impurities statedishouldrnot: simultaneously be presentgat their stated maximum amounts. The total of all. of the foregoing impurities,ytogether, with such other impurities as imay-bepresent, should not exceed about2%. 3. I. The following table contains data with respect to the creep strengtl'!- of various alloys "within and a without our invention. Each ofjthegfive alloys set forthin Table I below rco a d ut 2 chr mium. molybdenum, the amounts of carbon and cobalt respectively According to this test a piece of the alloy is cast in such manner as to have an acute-angle edge, the contained angle between the two flat sides of the piece being about 25. The edge itself is roundedso that the distance between the points at each side where the rounded edge merges intoit-he-flat side of the piece is approximately .062 inches. In each cycle of the test, such a piece is raised in temperature from room temperature to the in-T. dicated maximum temperature of the particular test (i. e. 850, 950 C., etc.) at an average rate of about 30 to 50 C. per second,held at the indicated maximum temperature for five seconds, and then cooled at approximately the same rate. by aspray of water or compressed stated for, each, and the balance essentially, all nickel plus impurities 'as above described:

TABLE I Efiect on creep. srrength of variations in carbon and cobalt Tho expres sionfit. s. i. means long tons (M 2240 lbs. each) per square inch.

Of the foregoingexamples alloys" Nos. 3, 4, S and 6 are within" the scopeof our invention having an hours to fracture characteristic under the stated conditions of at least about 30 hours. 9 0n the other hand, alloys Nos. 1 and 2 are not. within the scope of our invention. In each of these the carbon is toolow, and in the case of alloy No. 1 the cobalti's alsotoo low. .Alloys Nos. 3, 4 and S are within the preferred range of our alloy in that1the hours to fracture characteristic is at least about 50 hoursor more; The best formof our alloy known to us is approximatelythat indicated by alloy No. 4, which has an hourto fracture characteristic under the stated condition above 350 hours;

The deleterious efiects of the addition of excessive amounts of theimpurities aluminium and titanium are indicated in Table II below; The alloys referred to in this table consist of about 21% chromium, 10% molybout at various maximum temperatures.

denum, the amounts of carbon, cobalt, aluminium and titanium respectively set forth in the table, andthe balance essentially nickel plus residual impurities. Alloy No. 5 in this table is the same as the alloy of the same V number in Table I.

" f "TABLEII if 1 Efle ct or: creep strength of addition of l aluminium and titanium The superior thermal shock resistance of our alloy, in comparison with that of other known alloys heretofore used in the nozzlevanes of internal combustion turbine engines, was shown by thefollowing thermal shock test, by which the thermal shock index for the alloy was determined air, or both, to room temperature. The spray is maintained'for a period of about forty seconds, and the cycle is then repeated. In these tests the thermal shock index for the designated temperature is the number of complete .cycles reaching that temperature which the piece will withstand before appearance .of a crack visible to the naked eye extending completely across the rounded edge from one side or point of tangency to the other.

As shown in Table III below, these tests were carried The alloys used in these tests for comparison with the alloy according to the present invention are alloys which have heretofore been used in the guide vanes and turbine blades of internal combustion turbine engines.

TABLE I11 Thermal shock index at various maximum temperatures [Number of cycles'to crack] 850 0. 950 C. l,050 C. 1,l50 C.

1 Test suspended, no sign of cracking or other failure.

The alloys used in. the above tests were as follows:

Alloy No. 8 was the alloy known in Great Britain as H. R. Crown Max which, until about November 1953, was in regular use in the guide vanes of Rolls-Royce Dart? engines. This alloy," which does not contain molybdenum or cobalt, consists approximately of 0.15 to 0.30% carbon, 0.75 to 2% silicon, a maximum of 1% manganese, 10 to 15% nickel, 20 to 25% chromium, 2.5 to 3.5% tungsten, and the remainder (over 50%) iron.

The alloy referred to aboveuas alloy No. 9 is the alloy sold inGreat Britainunder the name Nimonic which has beenwidely used in the turbine blades of internal combustion: turbine engines. It does not contain any molybdenum. Its composition is approximately not more than 0.10% carbon, not more than 1.5% silicon, not more: than 1%-manganese,r not more than 5% iron, 0.8 to 2% aluminium, 1.8' to 3% titanium, 18 to 21% chromium, 15 to 21% cobalt, and thebala'nce (about 50%) nickel.

The alloy referred to above as. alloy No. 10 is the alloy known: commercially. in Great Britain as C.l30 which since November 1953 has been in regular use in the nozzle guide vanes of Rolls-Royce Dart engines.

' This alloy does not contain any appreciable amount of cobalt nor more thanrtrace amountsof carbon and relies upon the presence of substantial amounts of aluminium and titanium. Such alloy consists of about 21 to 23% chromium,,9.5 to 0.5% molybdenum, 0.7 to 1% aluminium, 2.4 to 2.9% titanium, not more than 0.06% carbon, not more than 0.75% iron, notmore than 0.60%

silicon, not-more than 0.60% manganese, no cobalt, and

the balance (nearly 60%) nickel plus traces-of impurities.

Alloy No. 11, which was made according to the present invention, *oonsists of about 21% chromium, 9.6%

molybdenum, 0.35% carbon, cobalt, and the balance essentially nickel plus impurities as above described.

The alloy referred to above as alloy N0. 12 is the alloy known commercially as Stellite 21 and commonly used in the United States as a casting material for vanes and blades. This alloy is of cobalt base and its composition is approximately 0.2 to 0.3% carbon, 1% max. manganese, 1% max. silicon, 0.007% max. boron, 3% max. iron, 25 to 29% chromium, 1.75 to 3.75% nickel, 5 to 6% molybdenum, and the remainder cobalt.

The alloy No. 13 referred to above is that known as Stellite No. 31 and is also commonly used in the United States for casting vanes and blades. It is of cobalt base and its composition is approximately 0.45 to 0.55% carbon, 1% max. manganese, 1% max. silicon, 0.04% max. phosphorus, 0.04% max. sulphur, 2% max. iron, 24.5 to 26.5% chromium, 9.5 to 11.5% nickel, 7 to 8% tungsten, and the remainder cobalt.

Our alloy may be prepared by any of the electric furnace melting processes which are well-known to those skilled in the art.

The alloy may be used as cast or in the forged condition by which we mean mechanically reduced in cross-section area. Stationary blades and vanes are preferably cast and turbine rotating blades preferably forged.

When forged a suitable heat treatment is solution treatment at 1200 to 1280 C. for a period of 1 to 4 hours, followed by air cooling and thereafter followed by an ageing heat treatment of 1 to 8 hours at 750 to 1000 C.; the part is again air cooled. Normally where the alloy is used in the cast condition it is not heat treated.

Percentages of composition throughout this specification are percentages by weight of the Whole alloy.

What is claimed is:

1. An alloy having a thermal shock index at 950 C. of at least about 60, and an hours to fracture characteristic under stress of 4480 lbs. per sq. inch at 1050 C. of at least about 30 hours, said alloy consisting essentially by weight of about 18 to 25% chromium, about 8.5 to 12% molybdenum, the impurities iron, aluminium and titanium not exceeding respectively about 0.75% of iron, about 0.2% of aluminium and about 0.3% of titanitun, all impurities including residual traces of cleansers and de-oxidizes not exceeding about 2%, and the balance nickel, said alloy being characterized in that it contains about 0.20 to 0.85% carbon and about 5 to 15% cobalt.

2. An alloy having a thermal shock index at 950 C. V

of at least about 60, and an hours to fracture characteristic under stress of 4480 lbs. per sq. inch at 1050 C. of at least about 30 hours, said alloy consisting essentially by weight of about 18 to 25 chromium, about 8.5 to 12% molybdenum, about 0.20 to 0.85% carbon, about 5 to 15 cobalt, and the balance nickel plus impurities.

3. An alloy having a thermal shock index at-950 C. of at least about 60, and an hours to fracture characteristic under stress of 4480 lbs. per sq. inch at 1050 C. of at least about 50 hours, said alloy consisting essentially by Weight of about 18 to 25 chromium, about 8.5 to 12% molybdenum, the impurities iron, aluminium and titanium not exceeding respectively about 0.75% iron,

about 0.2% aluminium and about 0.3% titanium, all impurities including residual traces of cleansers and deoxidizers not exceeding about 2%, and the balance nickel,

said alloy being characterized in that it contains about 0.20 to 0.60% carbon and about 5 to 11% cobalt.

4. An alloy having a thermal shock index at 950 C. of at least about 60, and an hours to fracture characteristic under stress of 4480 lbs. per. sq. inch at 1050 C. of at least about 50 hours, said alloy consisting essentially by weight of about 20 to 23% chromium, about 9.5 to 11% molybdenum, about 0.30 to 0.60% carbon, about 9 to 11% cobalt, and the balance nickel plus impurities.

5. An alloy having a thermal shock index at 950 C. of at least about 60, and an hours to fracture characteristic under stress of 4480 lbs. per sq. inch at 1050 C. of about 78 hours, said alloy consisting by weight of about 21% chromium, about 10% molybdenum, the impurities iron, aluminium and titanium not exceeding respectively about 0.75% of iron, about 0.2% of aluminium, and about 0.3% of titanium, all impurities including residual traces of cleansers and de-oxidizers not exceeding about 2%, and the balance essentially nickel, said alloy being characterized in that it contains about 0.20% carbon and about 5% cobalt.

6. An alloy having a thermal shock index at 950 C. of at least about 60, and an hours to fracture characteristic under stress of 4480 lbs. per sq. inch at 1050 C. of about 133.3 hours, said alloy consisting by weight of about 21% chromium, about 10% molybdenum, the impurities iron, aluminium and titanium not exceeding respectively about 0.75% of iron, about 0.2% of aluminium, and about 0.3% of titanium, all impurities including residual traces of cleansers and de-oxidizers not exceeding about 2%, and the balance essentially nickel, said alloy being characterized in that it contains about 0.58% carbon and about 9.72% cobalt.

7. An alloy having a thermal shock index at 950 C. of at least about 60, and an hours to fracture characteristic under stress of 4480 lbs. per sq. inch at 1050 C. of

, about hours, said alloy consisting by Weight of about 21% chromium, about 10% molybdenum, the impurities iron, aluminium and titanium not exceeding respectively about 0.75% of iron, about 0.2% of aluminium, and about 0.3% of titanium, all impurities including residual traces of cleansers and de-oxidizers not exceeding about 2%, and the balance essentially nickel, said alloy being characterized in that it contains about 0.75% carbon and about 9.55% cobalt.

8. An alloy having a thermal shock index at 950 C. at least about 60, and an hours to fracture characteristic under stress of 4480 lbs. per sq. inch at 1050 C. of at least about 30 hours, said alloy consisting essentially by weight of about 21% chromium, about 9.6% molybdenum, the impurities iron, aluminium and titanium respectively about 0.75 of iron, about 0.2% of aluminium, and about 0.3% of titanium, all impurities including residual traces of cleansers and de-oxidizers not exceeding about 2%, and the balance nickel, said alloy being characterized in that it contains about 0.35% carbon and about 10% cobalt.

References Cited in the file of this patent UNITED STATES PATENTS 2,712,498 Gresham et al. July 5, 1955 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No 2 840 469 June 24 1958 Harold Eh Gresham et a1,

It is herebjr certified that error appears in the-printed specification of the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 2,, line 37, after above" insert as about line 68, for "aluxcuirmin read aluminium a column 4, line 69, for "035%" read 10 5% a column 5, line 44, for de -oxitizes" read e ds'soxidizers column 6, line 21 for OD2O%" read O29%, line 52 after "titanium" insert not exceeding Signsd and sealed this 16th day of September 1958.,

(SEAL) Attest:

KARL Ha AICJINE ROBERT C. WATSON Commissioner of Patents Attesting Officer 

2. AN ALLOY HAVING A THERMAL SHOCK INDEX AT 950*C. OF AT LEAST ABOUT 60, AND AN HOURS TO FRACTURE CHARACTERISTIC UNDER STRESS OF 4480 LBS. PER SQ. INCH AT 1050*C. OF AT LEAST ABOUT 30 HOURS, SAID ALLOY CONSISTING ESSENTIALLY BY WEIGHT OF ABOUT 18 TO 25% CHROMIUM, ABOUT 8.5 TO 12% MOLYBDENUM, ABOUT 0.20 TO 0.85% CARBON, ABOUT 5 TO 15% COBALT, AND THE BALANCE NICKEL PLUS IMPURITIES. 